The present invention relates to vehicle transmission systems and is concerned with transmission systems for four wheel drive vehicles.
Automotive differentials are of course well known and include a single input shaft, normally connected to the output shaft of the vehicle gearbox, and two output shafts which are permitted to rotate at different speeds. The mean of these two speeds is directly proportional to the input speed. When a vehicle travels on a radius, that is to say goes round a corner, the outer wheels travel further than the inside wheels in the same time and must therefore rotate faster. The basic differential caters for this speed difference but the torque supplied to both wheels remains the same. However, there are many circumstances in which it is desirable for the torque supplied to the two wheels to differ. Thus if one wheel is on a low friction surface, e.g. ice, and the other on a high friction surface, e.g. a dry patch of roadway, the maximum torque that can be transmitted to the two wheels by a conventional differential is limited to the very low torque value which may be transmitted to that wheel which is in contact with the ice. The potential propulsive force available through the wheel in engagement with the high friction surface cannot be used and the propulsive force exerted may therefore be insufficient to move the vehicle. A vehicle cornering with a high lateral acceleration has a significant weight transfer from the inside wheels onto the outer wheels. As a result of the reduced weight on the inside wheels, only a low torque can be transmitted through the inside driven wheel before wheel slip occurs and this means that, with a conventional differential, only the same low torque may be transmitted to the outer driven wheel. This severely limits the potential acceleration of the vehicle, when cornering. In order to overcome these disadvantages, numerous devices are known to lock or limit slip in the differential, whereby different levels of torque may be transmitted through the two output shafts.
In the case of a four wheel drive vehicle, there is conventionally a differential between the front and rear axles to accommodate the speed differences between them whilst transmitting power to both axles. In this case also there are many circumstances under which a simple differential is inadequate. A four wheel drive facility is generally provided on vehicles with an excess of power and superior performance could therefore be achieved by transmitting the maximum amount of power through all the wheels. The maximum rate of acceleration can only be achieved if the torque is proportioned between the wheels to match the weight distribution of the vehicle. As a result of dynamic weight transfer, e.g. under heavy acceleration, as much as 80% of the weight may be carried by the rear wheels. An unequal torque distribution could be achieved by providing a differential with an invariable torque split to match this proportion but when cornering or driving on an icy road this torque split ratio could result in vehicle instability and thus loss of control. Under these conditions it would be advantageous for a higher proportion of the torque to be provided to the front axle.
In the case of xe2x80x9coff roadxe2x80x9d vehicles, which frequently have a short wheel base and a high centre of gravity, the weight transfer when climbing a steep gradient could be as much as 90% onto the rear axle. A limited slip type of differential could deliver sufficient traction but could also cause slippage of the front wheels, thereby leading to loss of stability. Locking the centre differential, that is to say the differential between the front and rear axles, would prevent front wheel slip but reduces manoeuvrability which is a significant problem on the poor surfaces which are commonly encountered when driving off road.
When descending a steep gradient, the safest technique is to use a low gear ratio and rely solely or principally on engine braking to maintain control. Under these circumstances, the weight transfer that occurs may place as much as 90% of the vehicle weight onto the front axle.
It is therefore the object of the invention to provide a transmission system for four wheel drive vehicles which will split the engine torque between the front and rear axles in a proportion which is variable in dependence on one or more operating parameters of the vehicle or its engine rather than merely limiting the slip of or locking the differential.
According to the present invention, there is provided a transmission system for a four wheel drive vehicle including an input shaft connected to a differential mechanism, which has two output shafts, which are connected, in use, to respective pairs of driven wheels of the vehicle, the two output shafts carrying first and second coaxially mounted sun wheels, respectively, of an epicyclic gear system which mesh with first and second sets of planet wheels, respectively, the epicyclic gear system including a third sun wheel, which is mounted coaxially with the first and second sun wheels and is in mesh with a third set of planet wheels, the gear ratios of the first sun wheel with the first set of planet wheels, the second sun wheel with the second set of planet wheels and the third sun wheel with the third set of planet wheels being different, each first planet wheel being connected to respective second and third planet wheels to rotate therewith about a respective common planet shaft, the planet shafts being connected to a common carrier which is rotatably mounted coaxially with the first, second and third sun wheels, the carrier being connected to a first selectively operable speed changing device, the third sun wheel being connected to a second selectively speed changing device, the transmission system further including at least one sensor arranged to produce a signal indicative of an operating parameter of the vehicle or its engine and a controller connected to the sensor and to the two speed changing device and arranged to operate the speed changing devices in response to the said signal. The present invention also embraces a four wheel drive vehicle incorporating such a transmission system.
Thus a four wheel drive vehicle in accordance with the present invention will typically include not only a front differential splitting speed and/or torque between the two front driven wheels, a rear differential splitting speed and/or torque between the two rear driven wheels and a centre differential splitting speed between the front and rear axles but also an epicyclic gear system, which is connected to the two outputs of the centre differential and includes two speed changing devices which are actuated by a controller in response to a signal received from one or more sensors which detect respective operating parameters of the vehicle or its engine to split the engine torque between the front and rear axles in a proportion which is the optimum for the prevailing operating conditions.
Thus in the transmission system in accordance with the invention there are two speed changing devices which are arranged to change the speed of the carrier and the third sun wheel, respectively, thereby varying the proportion of the engine torque supplied to the front and rear axles of the vehicle. The speed changing means may be arranged to increase or decrease the speed of the carrier and the third sun wheel. They may therefore constitute e.g. electric motors. It is, however, preferred that the speed changing devices are arranged to reduce the speed of rotation of the carrier and the third sun wheel and may thus constitute simple braking mechanisms. It is, however, preferred that they constitute clutches.
Further features and details of the invention will be apparent from the following description of two specific embodiments of transmission systems in accordance with the invention which is given by way of example only with reference to the two accompanying highly diagrammatic drawings.